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equinox 1. One of the two points of intersection of the ecliptic and celestial equator, occupied by the sun when its declination is 0°. That point occupied on or about March 21, when the sun's declination changes from south to north, is called vernal equinox, March equinox, or first point of Aries; that point occupied on or about September 23, when the declination changes from north to south, is called autumnal equinox, September equinox, or first point of Libra. Also called equinoctial point. 2. That instant the sun occupies one of the equinoctial points. Equinox means "equal nights," since the duration of daylight (sunrise to sunset) is exactly equal to the duration of darkness (12 hours) everywhere on earth on the equinoxes. Also, since the declination of the sun is 0° on the equinoxes, the sun always rises due east and sets due west on the equinoxes, viewed from everywhere on earth. See also solstice.

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